I was thinking of the accelerometer being located near the outside of the shell, rather than relatively stationary near the centre of rotation. I agree it's not entirely trivial, but it seems pretty doable to me.
The accelerometer cannot measure the earths gravity which was probably what the GP suggested. However you can measure the centrifugal force (http://xkcd.com/123/) which may be more precise than trying to measure the muzzle velocity or just assuming certain known muzzle velocity.
